1. Field of the Invention
This invention relates to unitary, self-generating reference gas sensors, useful to monitor not only a SO.sub.2, CO.sub.2 or NO.sub.2 component, but also an O.sub.2 gas component of a monitored gas environment.
2. Description of the Prior Art
The requirements for monitoring and controlling stack gas pollutants have resulted in the development of solid electrolyte gas sensors having electrolyte compositions uniquely responsive to gases such as SO.sub.2, CO.sub.2 and NO.sub.2. These sensors are electrochemical concentration cells which sense the equilibrium of a gas species of interest and generate an EMF signal corresponding to the difference in partial pressure of the gas species across the solid electrolyte sensor. Typically, the solid state sensor includes an ion conductive solid electrolyte with electrodes disposed on its opposite surfaces. The stack gas, or monitored gas stream, contacts a sensing electrode, while the opposite electrode serves as a reference electrode which is contacted with a reference gas stream. Conventional solid electrolyte compositions require operating temperatures of between 200.degree. C. and 900.degree. C. to exhibit the desired ion conductivity to generate a suitable EMF signal.
In the past, a major problem with these devices was isolation of the monitored gas from the reference gas, to prevent unpredictable drift in the measurement signal. In an attempt to not only effectively seal monitored gas from the reference gas, but to also eliminate the effect of O.sub.2 on the EMF signal measurement of SO.sub.2, U.S. Pat. No. 4,391,690, (Lin et al.) taught construction of a dual gas monitoring sensor device. Different and separate sensor cells were described: an SO.sub.2 cell having a K.sub.2 SO.sub.4 solid electrolyte, which is fed a SO.sub.2 reference gas stream, and is also connected to a source of O.sub.2 ; and an O.sub.2 cell having an oxygen ion conductive solid electrolyte, which is fed an air reference gas stream. This sensor design, however, is complicated to make and operate. Also, the use of a SO.sub.2 +O.sub.2 reference gas stream, is inconvenient and expensive, since a constant supply of certified tank gas is needed.
Several instances of simplified, unitary gas sensors have been disclosed in the art. In U.S. Pat. No. 3,915,830, (Isenberg) relating to O.sub.2 sensors, taught hermetically encapsulating a metal/metal oxide reference medium, such as nickel/nickel oxide, exhibiting a stable oxygen activity, within a small, stabilized zirconia solid electrolyte disc. U.S. Pat. No. 4,394,240, (Pebler) taught triangular, combination, multisensor electrochemical cells, which form an internal cavity which contains a common internal gas forming reference. In the triangular configuration, two sides were made of stabilized zirconia, oxygen ion conductive solid electrolyte and measure partial pressure of O.sub.2, and the third side could be made of K.sub.2 SO.sub.4 solid electrolyte when the partial pressure of SO.sub.3 or SO.sub.2 gases are to be measured. Reference electrodes were disposed on the inside electrolyte walls of the triangular configuration and sensing electrodes were disposed on the outside electrolyte walls.
The measuring concept in the Pebler patent utilized heating a central, enclosed, MgSO.sub.4, MnSO.sub.4 or Ag.sub.2 SO.sub.4 reference material, which provides SO.sub.3 on decomposition. This reference material had to be kept sealed from K.sub.2 SO.sub.4 electrolyte, because of the possible reaction of these two components at high temperatures. Each of the three cells had its own circuitry. Two cells were exposed to flue gas, and one of the zirconia cells is exposed to an environment of known oxygen partial pressure, such as air.
In an attempt to provide a simple, inexpensive construction that would be effective to measure SO.sub.2, CO.sub.2 or NO.sub.2 content of flue gases, U.S. Pat. No. 4,828,672 teaches a simplified, inexpensive, unitary, self-generating reference gas sensor. There, a reference electrode is isolated from the monitored gas environment by solid electrolyte, and the solid electrolyte itself, upon the application of heat, is effective to dissociate and provide the sole source of a self-generated gas, such as SO.sub.2 +O.sub.2, CO.sub.2 +O.sub.2, or NO.sub.2 +O.sub.2 at the reference electrode. That design could measure only SO.sub.2 +O.sub.2, CO.sub.2 +O.sub.2, or NO.sub.2 +O.sub.2, so that a separate O.sub.2 sensor would have to be installed along with the Lin et al. sensor, and the O.sub.2 concentration, in terms of voltage output, would have to be compensated for electronically.
In an attempt to provide a self-referencing gas sensor useful to monitor not only a SO.sub.2, CO.sub.2 or NO.sub.2 component, but also an O.sub.2 gas component, U.S. Pat. No. 4,828,671 (Lin et al.) teaches a stabilized zirconia cup which contains a solid mass of K.sub.2 SO.sub.4, K.sub.2 CO.sub.3, or KNO.sub.3, and the like electrolyte, where the zirconia is a O.sub.2 -cell and the K.sub.2 SO.sub.4 was an SO.sub.2 -cell. An inner Pt-electrode is sealed in the zirconia cup and embedded in the electrolyte, and is used as a common reference electrode. The K.sub.2 SO.sub.4 or like material is press-sintered at 100.degree. C. below its melting point within the zirconia cup to provide a solid, fused, electrolyte mass. It was found, that upon cooling, the K.sub.2 SO.sub.4 or like material could, in certain instances exhibit a large volume change during cooling to solidification which could lead to cracks in the electrolyte. What is needed is a further advanced design to provide a completely sealed, combined SO.sub.2 /O.sub.2, unitary, self-referencing gas sensor. It is one of the main objects of this invention to provide such a construction.